This invention relates to electron beam apparatus and in particular to such apparatus where secondary electrons from a sample being bombarded by a focussed electron beam are collected for the purpose of studying the properties of the sample giving rise to the production of those electrons.
The collection of electrons from a sample is often performed by drawing those electrons away from the sample in the general direction of the incident electron beam and then by means of suitably placed electrodes deflecting the electrons away from the beam towards some kind of collector, such as, for example, a scintillator and photomultiplier. The electrostatic field causing the relatively slow moving electrons from the sample under examination to be deflected towards the scintillator has a greatly reduced effect on the incident electron beam because its constituent electrons are travelling at a much higher speed due to the high voltages used to generate the beam and usually the effects on the beam are ignored. However, amongst the effects of this deflection field on the incident beam is a slight astigmatism which results in what should be a circular spot becoming elongated. This elongation of the spot arises apparently from the effect of the boundary surface of the electrostatic deflection field used for deflecting the electrons from the sample, which acts as a slightly "cylindrical" lens. Another cause for the distortion of the spot is the spread of speeds of the electrons constituting the incident beam which has the result, analogous to chromatic aberration, that different electrons are deflected by the same field to a different extent depending on their speeds.
It is an object of the present invention to provide improved electron beam apparatus in which the disadvantage described above is reduced.
According to the present invention there is provided electron beam apparatus in which the electron beam is directed to a sample and secondary electrons from the sample are returned in the direction of the beam and then deflected away from the beam by first electrostatic deflection means to an electron collector, wherein further electrostatic deflection means effective on the beam are located upstream of the first deflection means and the further electrostatic deflection means are so energised as to counteract the effect on the beam of the first deflection means.
Each of the first and further electrostatic deflection means may be a 4-element electrostatic stigmator in which one of the elements of the first means is a laterally extending tube with an electrically conducting grid over its inner end nearer the electron beam, and the voltages applied to the elements of the first means are such that the secondary electrons are deflected through the grid into the tube. A scintillator held at a high positive potential, say, 10kv, may be located at the outer end of the tube to receive the electrons and a photomultiplier may be placed to receive the light flashes from the scintillator. The voltages on the first deflection means should be more positive than the most positive voltage of the sample, so that the electrons from the sample are drawn towards the deflection means and can subsequently be directed to the collector. The further deflection means should have voltages which are sufficiently negative to reflect the electrons from the sample which might otherwise tend to pass the first deflection means. The further deflection means may produce a deflecting electrostatic field which has the opposite effect on the electron beam from that of the first deflection means. In practice, the voltages on the further deflection means needed to produce the compensating deflection of the beam can be chosen to be the correct voltages for reflecting the electrons from the sample back to the collector.
In order that the invention may be fully understood and readily carried into effect it will now be described with reference to the accompanying drawings, of which:
FIG. 1 is a diagram of one example of the electron beam apparatus according to the invention;
FIG. 2 is a perspective diagram of the first and further deflection means of the apparatus of FIG. 1; and
FIG. 3 shows a modification of the apparatus of FIG. 1.
The apparatus shown in FIG. 1 has an electron gun 1 which produces a beam 2 of electrons focussed on a sample 3 placed on a single-pole magnetic lens 4. Secondary and/or backscattered electrons come from the sample 3 as a result of the bombardment by the incident electron beam 2 and a hypothetical path of one of these electrons is indicated by the line 5.
An electron collector is provided and includes an electrically conducting tube 6 directed radially relative to the construction of the apparatus which has a central axis close to which the electron beam 2 runs. The inner end of the tube 6 has an electrically conductive mesh 7 through which the electrons from the sample 3 are drawn, which then pass down the tube 6 towards a scintillator 8 maintained at a high positive potential such as, for example, 10 kv. A photomultiplier 9 produces amplified signals from the light flashes from the scintillator 8 resulting from the impact of the electrons thereon. An electrode 10 also of cylindrical form is placed opposite the electron collector and is maintained at a positive potential slightly below that of the collector itself, so that the electrons are deflected into the collector. Both the collector and the electrode 10 should be maintained at potentials which are positive with respect to the most positive voltage on the part of the sample 3 under examination so that the electrons from the sample 3 are drawn towards the collector.
The electron collector and the electrode 10 are supported in a structure 11 so as to be insulated from it and from each other, the structure 11 being arranged to contain an upwardly directed magnetic field which is at its most powerful at the sample 3 and weakens progressively with distance from the sample 3, so that electrons from the sample 3 are contained by this field and move upwardly in helical paths of increasing pitch. A grid 12 located just below the electron collector and the electrode 10 may be provided to give a threshold, adjustable by the voltage applied to it, for the energy of the electrons from the sample 3 which pass the grid and are detected by the collector.
Although it is not shown in FIG. 1, an extractor grid maintained at a very high positive potential of 1000 volts, say, may be located at the lower end of the structure 11 to accelerate electrons leaving the sample 3 into the structure 11. A further grid or other means for producing an electrostatic field may also be provided for slowing down the accelerated electrons so that they approach the grid 12 at speeds substantially equal to those at which they left the sample 3.
As thus far described, the apparatus of FIG. 1 is similar to that already proposed in U.S. Pat. No. 4,658,137 (Simon C.J. Garth et al which is assigned to Texas Instruments Incorporated and incorporated herein by reference for electron beam apparatus used to examine the potentials of an integrated circuit when in operation and may incorporate details disclosed in that application.
The electron collector and the electrode 10 may be two elements of a 4-element electrostatic stigmator of which one of the other two electrodes is indicated at 13. Above the electron collector and the electrode 10, upstream relative to the direction of the electron beam 2, there is placed a second electrostatic stigmator consisting of four elements of which the elements 14, 15 and 16 are shown. The elements of the second stigmator are maintained at potentials which are sufficiently negative to reflect any electrons from the sample 3 back towards the collector taking into consideration their starting potentials and the energy they acquire accelerating to the first stigmator. In addition, the electrodes 14 and 16 are so biassed that they deflect the electron beam 2 in the opposite direction to the deflection produced by the electron collector and the electrode 10. The voltages on the other electrodes of the two stigmators are selected to reduce as far as possible the astigmatism of the spot produced by the electron beam resulting from the electrostatic field used to collect the electrons from the sample 3.
In a typical example, the electron collector, that is to say the tube 6 and the mesh 7, are maintained at +50 volts and the electrode 10 at +40 volts. If the voltage of the sample 3 were, say, 10 volts, this would mean that the potentials of the electrodes 16 and 14 respectively of the second stigmator should be -30 volts and -20 volts to ensure reflection of the electrons from the sample 3. The energy of the electron beam 2 itself may be, for example, 1-5 kv so that the deflection effects of the stigmators will be very much less on the electron beam than on the electrons from the sample 3 which will have an energy corresponding to 40 volts, that is to say some 25-125 times smaller. This will be the case for any electron beam system in which the energy of the electron beam is significantly greater than the energy of the secondary electrons.
FIG. 2 is a perspective diagram of the electrodes of the two stigmators shown in the apparatus of FIG. 1 which will enable the construction of the apparatus to be more clearly envisaged.
FIG. 3 shows a modification to the construction of the two stigmators to improve the collection of electrons by the electron collector. In this example, the electron collector consists of a tube 20 of larger diameter than the other electrodes of the stigmators with a mesh 21 across its inner end. In addition, the tube 20 has its inner end disposed further from the axis of the apparatus to provide a larger volume for the deflection of the electrons from the sample 3 to be displaced from the axis of the apparatus towards the collector by the electrostatic field. In order to maintain the same electrostatic field on the axis the voltage on the tube 20 and mesh 21 should be raised and may be, for example, 100 volts instead of 50 volts as in the example described above. Moreover, the electrode 16 is replaced by an L-shaped electrode 22 which is cut away at its underside to provide even more space for the deflection of electrons towards the collector. The tube 20, because of its larger diameter, may provide sufficient electrostatic attraction to collect the electrons without the need for the mesh.
Although the invention has been described with reference to just two possible constructions for the apparatus, other constructions for achieving the same result will be apparent to those skilled in the art. For example, instead of using a single pole magnetic lens with the sample standing on the pole piece as shown in FIG. 1, other arrangements, magnetic and/or electrostatic, may be used to control the paths of the electrons emitted from the sample, and where such alternative arrangements are employed the planar threshold grid 12, which sets the minimum energy for electrons to be passed to the collector, may need to be replaced by a differently shaped grid. A buffer grid, not shown in the drawings, may be provided above the threshold grid 12, and maintained at a potential that is more positive than any part of the sample 3, for the purpose of ensuring that any electrons from the sample passing the threshold grid enter the control region of the lower stigmator at a reasonable speed and do not drift slowly into it to be collected by one of the three electrodes other than the collector 6.